Torque converters carry out the functions of absorbing torque fluctuations or increasing torque in order to transmit the motive force between the input/output elements through a fluid.
Since their transmission efficiency is lower than the conventional friction clutch, during running conditions where absorbing torque fluctuations or increasing torque are not necessary, the input/output elements of the torque converter are directly connected and put in a lock-up state.
In vehicles provided with an automatic transmission and a torque converter equipped with such a lock-up clutch, the torque converter is generally put in the lock-up state during coast running in order to increase the fuel cut period.
However when shifting from coast running to power running by depressing the accelerator pedal while the torque converter is maintained in the lock-up state, the torque converter may experience a torque shock due to sudden increase of input torque.
In order to suppress this torque shock, Tokkai Hei 8-233098 published in 1996 by the Japanese Patent Office discloses the unlocking of the torque converter when shifting from coast running to power running is performed.
However in this conventional technique, if shifting between coast running and power running is frequently repeated during driving, the torque converter will be repeatedly locked up and unlocked. Thus the clutch facing of the lock-up clutch will soon wear out and this will adversely affect to the durability of the torque converter.